The present invention relates to a print head for printing optical structures on a substrate comprising an ejection device for ejecting at least one droplet of a printing ink towards the substrate in order to build up the optical structure.
It is common knowledge that products provided with optical structures can be manufactured in a comparably time-saving and inexpensive way by printing the optical structures directly on an appropriate substrate. Suchlike imprinted optical structures comprise lenses, mirrors or retroreflectors, for instance. The substrate can be made of synthetic material or glass. Furthermore, it is well known to cure the imprinted material by using light irradiation for reducing the overall printing time.
For example, PCT patent application WO 2010/091 888 A1 discloses a method for producing optical lenses from a moldable transparent material by depositing the material on a substrate in a layer that is subsequently cured by a laser or by UV irradiation. The transparent material is deposited on the substrate by a conventional printing-on-demand printer, for instance.
In order to provide high quality optical structures with certain optical effects, the three dimensional geometries and the surface contours of the optical structures have to be precisely designed according to a given pattern. Even the slightest deviation in the intended geometry of the optical structures results in significant optical errors. Consequently, the droplets have to be positioned onto the substrate with high accuracy and curing has to be performed depending on the present flow behavior of the printing ink.
This is very difficult because the behavior of the Just at the moment deposited and still uncured droplets strongly depends on many factors, like material and surface conditions of the substrate, composition of the printing ink, atmospheric pressure, temperature, air moisture, orientation between the printer and the gravitational field and the like. Disadvantageously, it is not possible to take into account all of these factors previously.